There exists a need for a blood-substitute to treat or prevent hypoxia resulting from blood loss (e.g., from acute hemorrhage or during surgical operations), resulting from anemia (e.g., pernicious anemia or sickle cell anemia) or resulting from shock (e.g., volume deficiency shock, anaphylactic shock, septic shock or allergic shock).
The use of blood and blood fractions as in this capacity as a blood-substitute is fraught with disadvantages. For example, the use of whole blood often is accompanied by the risk of transmission of hepatitis-producing viruses and AIDS-producing viruses which can complicate patient recovery or result in patient fatalities. Additionally, the use of whole blood requires blood-typing and cross-matching to avoid immunohematological problems and inter donor incompatibility.
Hemoglobin, as a blood-substitute, possesses osmotic activity and the ability to transport and transfer oxygen. However, aqueous hemoglobin exists in equilibrium between the tetrameric (65 KDa) and dimeric (32 KDa) forms. Hemoglobin dimers are excreted by the kidney and result in rapid intravascular elimination of hemoglobin solutions with such solutions typically having a 2-4 hour plasma half-life.
Efforts have been directed to overcome the inherent limitations of hemoglobin solutions by molecularly modifying the hemoglobin. Intramolecularly and intermolecularly cross-linking hemoglobin has generally reduced renal elimination and increased intravascular retention time.
However, solutions of cross-linked hemoglobin still typically contain a significant fraction of unmodified tetrameric hemoglobin. This unmodified tetrameric hemoglobin can convert to dimeric hemoglobin and then be excreted from the body, thereby reducing the average intravascular retention time for cross-linked hemoglobin blood-substitutes. Furthermore, current means for separation, such as standard filtration, do not adequately distinguish between unmodified tetrameric hemoglobin and modified tetrameric hemoglobin.
Thus, in spite of the recent advances in the preparation of cross-linked hemoglobin blood-substitutes, the need continues to exist for a method to effectively separate unmodified hemoglobin from a solution of an intramolecularly and/or intermolecularly cross-linked hemoglobin blood-substitute to improve the average intravascular retention time of the blood-substitute and to prevent significant levels of renal excretion of hemoglobin.
Prior approaches to removal of various impurities from hemoglobin solutions has focused on relatively low temperature long term (longer than one hour) heat treatment processes. U.S. Pat. No. 5,281,579 describes heat treatment from 45 to 85° C., and particularly 60-66° C. for 1 to 30 hours. U.S. Pat. No. 5,741,894 describes a process for removal of impurities from partially oxygenated hemoglobin solutions in a range of 45 to 85° C., and particularly 76° C. for 90 minutes. However, such long term heat treatment conditions can lead to the formation of met-hemoglobin, which cannot be used to oxygenate tissues. Further, such long term heat treatment processes are not compatible with commercial-scale production processes.